Revising. As long as you're going through topics at the right speed, you don't need to be learning stuff before you get to it in school, it just means you'll be bored later when you're being taught it again in school (It got a bit boring learning integration by parts 3 times)
I did textbook notes for Physics, condensed the textbook into ~10 pages along with tables for the definitions to memorise. I also read mark schemes and occasionally examiners reports, as they tell you exactly what you need to include in answers, and the reports tell you where everyone else screws up so you can focus on those bits/
For maths, past or mock exam questions.